A trial per pais is a trial by the country, i.e. by a jury; and matter in pais is matter triable by the country, or jury.

Part or all of this entry has been imported from the 1913 edition of Webster’s Dictionary, which is now free of copyright and hence in the public domain. The imported definitions may be significantly out of date, and any more recent senses may be completely missing.
(See the entry for pais inWebster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913.)

The vast majority of facsimiles of manuscripts use pais to mean peace and païs (with a diaeresis on the i) to mean country. While this avoids ambiguity this distinction is not found in the original manuscripts which do not contain diaereses at all.